1. Technical Field
The invention relates to inkjet printing. More particularly, the invention relates to ink jet UV pinning for control of gloss.
2. Description of the Background Art
Certain types of printing systems are adapted for printing images on large-scale substrates, such as for museum displays, billboards, sails, bus boards, and banners. Some of these systems use so-called drop on demand ink jet printing. In these systems, a carriage which holds a set of print heads scans across the width of the substrate while the print heads deposit ink as the substrate moves.
Solvent based inks are sometimes used in these systems in which an infrared dryer is used to dry off the solvent after the ink is deposited onto the substrate. Systems using solvent based inks are able to print on flexible substrates such as PVC materials and reinforced vinyl. However, solvent based inks are typically considered to be unusable for printing on rigid substrates such as metals, glass, and plastics. Therefore, to print on rigid, as well as flexible substrates, radiation-curable inks such as UV-curable inks are often preferred. For these systems, the ink is deposited onto the substrate and then cured in a post-printing stage. For instance, after the deposition of the ink, the substrate moves to a curing station. The ink is then cured, for example, by exposing it to UV radiation. In other systems, the UV radiation source for curing is mounted directly on the same carriage that carries the set of print heads.
UV ink jet dot gain is a parameter that is difficult to control. Ink deposited onto a substrate, until it is cured with UV energy, can react by spreading or shrinking depending on the surface tension and surface energy of the ink and substrate. Drop to drop interactions also complicate the control of dot gain and gloss. The time frames of interaction are such that locations of various colors and print heads with respect to the cure lamp result in differential gloss banding, an objectionable printing artifact.
Methods to correct this time-to-lamp problem have been proposed and implemented. For example, Ink Jet Printer with Apparatus for Curing Ink and Method (U.S. Pat. No. 6,145,979) describes a method to prolong, uniformly, the time-to-lamp for an ink jet printer through the use of mirrors or a post cure lamp traveling with the print carriage.
Image Forming Apparatus Having a Plurality of Printing Heads (U.S. Pat. No. 7,152,970) describes a method of positioning UV cure lamps adjacent to each print head color to equalize the time to lamp between print heads and colors.
Digital Ink Jet Printing Method and Apparatus and Curing Radiation Application Method (U.S. Pat. No. 7,837,319) describes a method of applying a first and second intensity UV cure energy, each applied at a constant time for all locations on the substrate.
Another method used to mitigate differential gloss banding is to use pinning (aka setting), the application of a low UV energy (the order of 5% of cure energy) to freeze or gel the ink dots on the media as soon as possible after application to the media, where they are later cured by high intensity UV radiation. Examples of this method are disclosed in Systems and Methods for Curing a Fluid (U.S. Pat. No. 6,739,716), which describes two UV cure lamps or reflectors that direct two different power levels onto a substrate as ink jet ink is applied. The result is to freeze each layer of ink that is applied so as to prohibit interaction between the ink layers.
Method of Printing Using Partial Curing by UV Light (U.S. Pat. No. 7,152,969) similarly describes pinning to allow many passes of ink application without drop to drop interaction.
The assignee of the present application, EFI, holds two patents in this area: Apparatus and Method for Setting Radiation Curable Ink (U.S. Pat. No. 6,457,823) and Radiation Treatment for Ink Jet Fluids (U.S. Pat. No. 7,600,867), both of which are aimed at inhibiting ink to ink or ink to substrate interactions.
Methods of controlling ink interactions to minimize gloss banding print artifacts by time-to-lamp or pinning and curing can still result in print artifacts due to other variables. Short times to lamp or pinning result in low dot gain with thick ink build up and loss of color due to small dot size. Gloss banding continues to persist due to bidirectional laydown of droplets, which result in physical reflectance which is directionally viewing dependent.